1. Field of the Invention
The present invention relates to thermal printing, and more particularly, to a thermal printer that can operate with both conventional linered and linerless print media.
2. Description of Related Art
In the field of bar code symbology, parallel bars of varying thicknesses and spacing are used to convey information, such as an identification of the object to which the bar code is affixed. To read the bar code, the bars and spaces are scanned by a moving light source, such as a laser, or imaged by an optically imaging element, such as a charge coupled device. Since the bars and spaces have differing light reflective characteristics, the information contained in the bar code can be read by interpreting the light that reflects from the bar code, or the image pattern that contains the bar code.
Bar codes are often printed onto a print media that can be associated with or affixed to the objects intended to be identified. The print media typically comprises a face material onto which the bar code is printed and an adhesive backing layer applied to an opposite surface of the face material that permits it to be affixed to an object. The face material may be further separated into discrete labels that are laminated onto a release liner having a low-stick surface that allows the labels to be removed easily. After the information is printed onto the face material, the user can simply peel off the individual labels from the release liner, and apply the labels onto an object of interest. Alternatively, the print media may include a non-adhesive card or tag stock having a texture similar to the release liner of the adhesive-backed print media. Such print media may include perforation lines to separate individual tags or cards following printing of information thereon, and a tear bar of the printer may further facilitate separation of the tags or cards. These various types of print media are referred to collectively herein as conventional print media.
In order to accurately read the bar code, it is thus essential that the bar code be printed in a high quality manner, without any streaking or blurring of the bar code, or misplacement of the bar code due to transportation of the print media at a non-uniform rate. Moreover, it is essential that the adhesive backing layer of the print media not be damaged by heat generated during the printing process, otherwise the media will not stick properly to the object. In view of these demanding printing requirements, bar codes are often printed using direct thermal or thermal transfer printing techniques. In direct thermal printing, the face material of the print media is impregnated with a thermally sensitive chemical that is reactive upon exposure to heat for a period of time. Thermal transfer printing requires an ink ribbon that is selectively heated to transfer ink to the face material. These printing techniques are referred to collectively herein as thermal printing. To print the bar code symbols, the print media is drawn past a thermal print head having linearly disposed printing elements that extend across a width of the print media. The printing elements are selectively activated in accordance with instructions from a controller to heat localized areas of the substrate or ink ribbon, thereby creating a dark image by a chemical reaction brought on by the heat. As the labels are drawn through a print region between a platen and the thermal print head, the bar code is printed onto the face material. Other images, such as text, graphics or characters, can also be printed in the same manner.
In a relatively new formulation of print media, the release liner is eliminated altogether, and the media is simply wound onto itself with the adhesive backing layer adhering directly to the face material of subsequent portions of the media. This so-called "linerless" print media includes an adhesive backing layer specifically formulated to prevent formation of a permanent adhesive bond, enabling the media to be subsequently peeled off as a roll of the media unravels without damaging the face material. Linerless media is more convenient than conventional media for certain types of applications, and by eliminating the release liner, a substantial amount of waste material normally generated in the labeling process is eliminated. Also, the elimination of the release liner allows additional print media to be wound onto the supply roll, which increases the number of labels that can be printed between roll changes. The linerless media may further include perforation lines similar to that of the tag stock media described above.
A significant drawback of the linerless print media is that the adhesive backing layer tends to form adhesive deposits on various internal surfaces of the transporting mechanism of the printer. These adhesive deposits cling to portions of the print media as it is transported thus increasing the friction of the transport path. Moreover, the adhesive deposits collect dirt, dust and other debris within the printer that can blemish the print media. As a result of these undesirable effects, the overall performance of the printer becomes degraded. While periodic cleaning of the printer mechanism ameliorates some of these effects, it also increases the unusable time of the printer as well as the maintenance cost of operating the printer.
One method to overcome this drawback is to coat certain portions of the printer transporting mechanism, including the platen, with an elastomeric substance. The elastomeric coating provides a glossy or smooth surface that resists bonding to the adhesive backing layer. Another method for printing on linerless media is illustrated in U.S. Pat. No. 5,560,293, which discloses a fineness label printer in which certain printer parts are plasma coated to provide a non-stick, low friction coating. As a result of these non-stick, smooth coatings, the linerless media can be effectively transported through the printer without forming the undesirable adhesive deposits. Although these coatings may enable linerless media to move through a printer without sticking, these coatings are known to cause slipping or frictional problems that produce misprinted barcodes when conventional media is used in the same printer. Thus, separate printers are generally required for printing on conventional and linerless media.
Thus, it would be desirable to provide a single thermal printer that is capable of efficiently operating with both conventional and linerless print media.